196 HISTOLOGY 



classes according to their nuclear characteristics, namely, into lymphocytes, 

 large mononuclear leucocytes, and polymorphonuclear leucocytes. 



Lymphocytes have already been briefly described with the constituents 

 of connective tissue (Fig. 56, p. 68). Ordinarily they are small cells, 

 about the size of red corpuscles, 4-7.5 M in diameter. Large ones may be 

 double this diameter. Their protoplasm forms a narrow rim, sometimes 

 almost imperceptible, about the dense round nucleus (Fig. 186, A). The 

 chromatin is arranged in a network associated with coarse chromatic 

 masses such as cause a characteristic checkered appearance. Some of the 



masses rest against the nuclear 

 membrane. Lymphocytes are 

 capable of amoeboid motion but 

 not to the extent of the poly- 



FIG. 186. LEUCOCYTES AS SEEN IN A SECTION OF Hu morphonuclear type. They 



MAN TISSUE PRESERVED WITH ZENKER'S FLUID. * 



A, Lymphocyte; B, large mononuclear leucocyte; C, form from 22 tO 25% of all 



three polymorphonuclear neutrophiles. , 



leucocytes. 



Large mononuclear leucocytes, sometimes 20 n in diameter, form only 

 from i to 3% of the leucocytes. They possess round, oval, slightly 

 indented, or crescentic nuclei, which are vesicular and usually eccentric 

 in position. Their chromatin occurs in a few large granules; as a whole 

 the nucleus is clear and pale (Fig. 186, B). The protoplasm, which is 

 abundant, usually lacks coarse granules or other distinctive features. 

 Sometimes it contains a few deeply staining granules as shown in one of 

 the cells in Fig. 187, II. The large mononuclear leucocytes are notably 

 phagocytic. In certain respects they are intermediate between lympho- 

 cytes and polymorphonuclear cells, and they were formerly known as 

 "transitional cells." Apparently, however, they are derived directly 

 from the modified endothelial cells lining the sinuses of the lymph glands, 

 and they have sometimes been regarded as the youngest of the forms of 

 cells shown in Fig. 186. 



Polymorphonuclear leucocytes are cells somewhat larger than red cor- 

 puscles, being from 7.5 to 10 n in diameter. They are characterized 

 by having nuclei with irregular constrictions leading to an endless variety 

 of shapes (Fig. 186, C). The nodular subdivisions may be connected by 

 broad bands or by slender filaments. It is said that in degenerating cells 

 the nucleus becomes divided into several separate masses. Such forms 

 can properly be called "polynuclear," an abbreviated term which is a 

 misnomer as applied to the ordinary cells; "mononuclear" as designating 

 the preceding types is also unfortunate since it implies that others have 

 several nuclei. The irregular shape of the polymorphous nuclei has been 

 ascribed to degenerative changes, comparable to those seen in the erythro- 

 blast nuclei. Within the concavity of the nucleus the centrosome may 

 be found, surrounded by a light area; usually it occurs as a diplosome. 



